1. Field of the Invention
This invention relates to multiple access communication systems and in particular it relates to dynamic resource allocation in time division multiple access systems.
2. Description of Related Art
In Multiple access wireless systems such as GSM, a number of mobile stations communicate with a network. The allocation of physical communication channels for use by the mobile stations is fixed. A description of the GSM system may be found in The GSM System for Mobile Communications by M. Mouly and M. B. Pautet, published 1992 with the ISBN reference 2-9507190-0-7.
With the advent of packet data communications over Time Division Multiple Access (TDMA) systems, more flexibility is required in the allocation of resources and in particular in the use of physical communication channels. For packet data transmissions in General Packet Radio Systems (GPRS) a number of Packet Data CHannels (PDCH) provide the physical communication links. The time division is by frames of 4.615 ms duration and each frame has eight consecutive 0.577 ms slots. A description of the GPRS system may be found in (GSM 03.64 V 8.5 release 1999). The slots may be used for uplink or downlink communication. Uplink communication is a transmission from the mobile station for reception by the network to which it is attached. Reception by the mobile station of a transmission from the network is described as downlink.
In order to utilise most effectively the available bandwidth, access to channels can be allocated in response to changes in channel conditions, traffic loading Quality of service and subscription class. Owing to the continually changing channel conditions and traffic loadings a method for dynamic allocation of the available channels is available.
The amounts of time that the mobile station receives downlink or transmits uplink may be varied and slots allocated accordingly. The sequences of slots allocated for reception and transmission, the so-called multislot pattern is usually described in the form RXTY. The allocated receive (R) slots being the number X and the allocated transmit slots (T) the number Y.
A number of multislot classes, one through to 29, is defined for GPRS operation and the maximum uplink (Tx) and downlink (Rx) slot allocations are specified for each class. The specification for multislot class 12 is shown in Table 1 below.
In a GPRS system, access to a shared channel is controlled by means of an Uplink Status Flag (USF) transmitted on the downlink to each communicating mobile station (MS). In GPRS two allocation methods are defined, which differ in the convention about which uplink slots are made available on receipt of a USF. The present invention relates to a particular allocation method, in which an equal number “N” of PDCH's, where a “PDCH” uses a pair of uplink and downlink slots corresponding to each other on a 1-1 basis, are allocated for potential use by the MS. The uplink slots available for actual use by a particular mobile station sharing the uplink channel are indicated in the USF. The USF is a data item capable of taking 8 values V0–V7, and allows uplink resources to be allocated amongst up to 8 mobiles where each mobile recognises one of these 8 values as ‘valid’, i.e. conferring exclusive use of resources to that mobile. In the case of the extended dynamic allocation method, for example, reception of a valid USF in the slot 2 of the present frame will indicate the actual availability for transmission of transmit slots 2 . . . N in the next TDMA frame or group of frames, where N is the number of allocated PDCHs. Generally for a valid USF received at receiver slot n, transmission takes place in the next transmit frame at transmit slots n, n+1 et seq. to the allocated number of slots (N). For the extended dynamic allocation method as presently defined these allocated slots are always consecutive.
The mobile station is not able instantly to switch from a receive condition to a transmit condition or vice versa and the time allocated to these reconfigurations is known as turnaround time. As presently defined the turnaround time depends upon the class of mobile. A turnaround time of one slot is allocated in the case of class 12 mobiles such as are used for the exemplary embodiment. It is also necessary for the mobile station, whilst in packet transfer mode, to perform neighbourhood cell measurements. The mobile station has continuously to monitor all Broadcast Control Channel (BCCH) carriers as indicated by the BA(GPRS) list and the BCCH carrier of the serving cell. A received signal level measurement sample is taken in every TDMA frame, on at least one of the BCCH carriers. (GSM 05.08 10.1.1.2)
These neighbour cell measurements are taken prior to re-configuration from reception to transmission or prior to re-configuration from transmission to reception. The number of slots allocated to each of these measurements and re-configurations for multislot class 12 is two.
Arising from the requirement to allocate particular slots for turnaround and measurement purposes, some restrictions occur and potential dynamic channel allocations are lost. These restrictions reduce the availability of slots for uplink transmissions; reduce the flow of data and reduce the flexibility of response to changing conditions.
An exhaustive technical review and wholesale change to the existing prescribed operating conditions might be expected to alleviate the problems associated with dynamic allocation. Whilst this is possible, the considerable difficulties caused by such wholesale change would be generally unwelcome and this resolution of the technical problem is unlikely.
There is a need therefore to provide a solution to the problems affecting dynamic channel allocation with minimal effect on existing prior art methods.